This invention pertains generally to amplifier stages and more particularly to a matched amplifier stage for use in high frequency applications such as microwave circuits.
At frequencies on the order of 200 Mhz or higher, transistors have very low input and output impedances, e.g. 1 ohm or less, which must be interfaced with the higher impedances of circuits in which the transistors are typically used. In the past, some transistors have been packaged with input and output matching networks in the transistor housing in order to raise the input and output impedances and thereby simplify circuit design.
Although successful commercially, transistors heretofore provided with integral matching networks are subject to certain disadvantages which will become more significant as the state of the art advances. Because of the space limitations imposed by present packaging techniques, output matching network connections are generally made to the transistor body at points spaced laterally from the signal output leads, and as a consequence there is a significant flow of output current in the transistor body and metalization attached thereto in a direction perpendicular to the normal current flow. This results in different phase angles for signal voltages along the transistor, with a corresponding reduction in efficiency. Moreover, since the body of most transitor dice is the collector, which is utilized as the output terminal, the thermal resistance of the device is limited because an electrical insulator, such as beryllia, must be used between the collector of the device and a grounded heat sink.